1. Field of the Invention
The present invention relates to a solid oxide fuel cell (SOFC, also called a solid oxide electrolyte cell). More particularly, the present invention relates to an air electrode of the cell and the structure of the air electrode, and a manufacturing method of the cell.
2. Discussion of the Background
The development of solid oxide fuel cells having a laminate structure, in which a solid electrolyte layer of oxide ion conductor material is interposed between an air electrode layer and a fuel electrode layer, represents a significant advance as a so-called third-generation power generating fuel cell.
In the solid oxide fuel cell, the air electrode is supplied with oxygen (for example air) and the fuel electrode is supplied with a fuel gas (H.sub.2, CO, or other gas). The air electrode and the fuel electrode are porous so that the gas reaches the interface with the solid electrolyte. Oxygen supplied to the air electrode moves close to the interface with the solid electrolyte through the pores in the air electrode, and receives there electrons from the air electrode, and is then ionized to oxide ions (O.sup.2-). Since the ionization of the oxygen molecule into the oxide ion in the air electrode involves oxygen molecules, electrons and oxide ions, the ionization takes place in only the three-phase interface of (1) a solid electrolyte layer transferring oxide ions, (2) air electrode particles transferring electrons, and (3) air for feeding oxygen molecules. The oxide ions move toward the fuel electrode in the solid electrolyte layer. The oxide ions diffuse through the solid electrolyte toward the fuel electrode. The oxide ions, close to the interface with the fuel electrode, react with a fuel gas, creating reaction products (H.sub.2 O, CO.sub.2, etc.), and discharging electrons to the fuel electrode. For the same reason, the above electrode reaction takes place in the fuel electrode in the three-phase interface only, where the solid electrolyte layer, the fuel electrode particles and a gaseous phase fuel gas meet. Expanding the three-phase interface is believed to be useful to smoothly carry out the electrode reaction.
When hydrogen is employed as a fuel, the electrode reaction is as follows. EQU Air electrode: 1/2O.sub.2 +2e.sup.31.fwdarw.O.sup.2- EQU Fuel electrode: H.sub.2 +O.sup.2-.fwdarw.H.sub.2 O+2e.sup.- EQU Overall cell: H.sub.2 +1/2O.sub.2.fwdarw.H.sub.2 O
(When CO is used, CO+1/2O.sub.2.fwdarw.CO.sub.2)
Since the solid electrolyte layer functions both as a transfer medium for oxide ions and a membrane for preventing the fuel gas from being in direct contact with air at the same time, the solid electrolyte is required to have a compact construction so as to be impermeable to the penetration of gas.
The solid electrolyte must be constructed of a material which shows a high ion conductivity, and is chemically stable under varying conditions from an oxidation environment in the air electrode to a reduction environment in the fuel electrode, and insusceptible to thermal shocks. As a material meeting these criteria, stabilized zirconia with yttria added (YSZ) is typically used for a solid electrolyte material. Stabilized zirconia has a crystal structure of face centered cubic lattice of the fluorite type. The conventional solid oxide fuel cells are operated at or in the vicinity of a temperature of 1000.degree. C., where stabilized zirconia, used as an electrolyte, exhibits a high oxide ion conductivity.
The air electrode (i.e., a cathode) and the fuel electrode (i.e., an anode) need to be constructed of a material having a high electron conductivity. Metals are not appropriate, since the air electrode material is required to be chemically stable in a high-temperature, oxidation environment at or in the vicinity of 1000.degree. C. Typically employed for the air electrode are perovskite type oxide materials having electron conductivity, for example, LaMnO.sub.3 or LaCoO.sub.3, or a solid solution of these with part of La replaced with Sr, Ca or the like. Although LaCoO.sub.3 outperforms LaMnO.sub.3 in terms of both polarization characteristics and electron conductivity, LaMnO.sub.3 is more widely used because LaMnO.sub.3 is similar to stabilized zirconia in thermal expansion coefficient. The material of the fuel electrode is typically a metal such as Ni or Co, or a cernet such as Ni--YSZ or Co--YSZ.
The solid oxide fuel cells are available in two types, namely, a cylindrical type as shown in FIG. 1A and a flat type as shown in FIGS. 1B(1)-1B(3).
The cylindrical unit cell shown in FIG. 1A is constructed by coaxially and compactly wrapping, around an insulating porous ceramic cylindrical body 1, an air electrode layer 2, a solid electrolyte layer 3, and a fuel electrode layer 4 in that order from the inside. A conductive interconnector 5, serving as a terminal for the air electrode 2, is in contact with the air electrode 2 but out of contact with the fuel electrode 4, and penetrates the electrolyte layer 3. Each layer is formed through a flame spraying process, an electrochemical deposition process, a slip casting process, or other process.
The single flat cell shown in FIGS. 1B(1)-1B(3) includes an air electrode layer 2 on one side of a solid electrolyte layer 3 and a fuel layer 4 on the other side of the solid electrolyte layer 3. Unit cells are mutually interconnected using fine-structured interconnectors 5, each having a gas passageway. The flat unit cell is produced as follows: a solid electrolyte layer is formed by sintering a green sheet that is produced by a doctor blade process or an extrusion process; a slurry of air electrode material is applied on one side of the solid electrolyte layer; a slurry of fuel electrode layer is applied on the other side of the solid electrolyte layer; and the laminated electrolyte layer is sintered. The sintering process may be performed sequentially or in an all-at-once fashion, in other words, the application of the slurry to each side is immediately followed by the sintering operation or the sintering operation is performed together on both sides after the slurry is applied onto both sides. Alternatively, the green sheets of the electrolyte layer and the electrode layers are produced and laminated, and are then subjected to an all-at-once sintering process. Such a wet process is less costly. In the same way as in the cylindrical type, the flame spraying process or the electrochemical deposition process may also be utilized.
In the solid oxide fuel cell thus constructed, the electrolyte layer is different from electrode layers in terms of elements and crystal structure. The distribution of an element is discontinuous, in other words, stepwise changes in the interface between the electrode layer and the electrolyte layer. Because of a difference in thermal expansion coefficient between materials, a unit cell is subject to a distortion, cracks, or delamination under thermal stress during the manufacturing stage such as in the sintering process or during the operation of the fuel cell. The elements diffuse between the materials or react during the sintering process or the operation, causing a high-resistance compound to develop in the interface, and thereby increasing interface resistance and degrading the bonding of the interface.
To resolve this problem, an air electrode is conventionally formed by mixing a perovskite type oxide, as an air electrode material, with an electrolyte material YSZ (see Japanese Unexamined Patent Publication 4-101359 and Japanese Unexamined Patent Publication 5-151981). This arrangement alleviates a sharp change in the thermal expansion coefficient in the vicinity of the interface between the electrolyte layer and the air electrode layer, resulting an improved bond. For the fuel electrode, the cermet of a metal and an electrolyte, such as Ni--YSZ, is employed for the same reason.
As disclosed in Japanese Unexamined Patent Publication No. 5-151981, a perovskite type oxide coexists with YSZ in the air electrode which is a mixture of the perovskite type oxide and YSZ, and a three-phase interface, needed to ionize oxygen molecules, is expanded. In other words, YSZ, mixed into the air electrode, works as aggregates to expand the threephase interface. In this sense, the mixing of the electrolyte material into the air electrode is effective. However, the interface between the electrolyte layer and the air electrode layer is still discontinuous in composition and construction, and the rates of change in the thermal expansion coefficient and composition are merely slightly reduced. This arrangement is not considered as a solution to fully resolving the above problem, and the effectiveness of the arrangement is not sufficient.
Several techniques have been proposed to reduce a difference in thermal expansion coefficient or composition between a solid electrolyte layer and an electrode layer (an air electrode layer, for example). For example, Japanese Unexamined Patent Publication No. 7-296838 discloses a layer, interposed between the a solid electrolyte layer and an electrode layer, and having a thermal expansion coefficient intermediate between those of the two layers. Japanese Unexamined Patent Publication No. 4-280075 discloses a layer constructed of mixed materials of the two layer. Japanese Unexamined Patent Publication No. 278663 and Japanese Unexamined Patent Publication No. 266000 respectively disclose a gradient composition layer in which a composition continuously changes between the two layers. Japanese Unexamined Patent Publication No. 5-29004 has proposed making a solid solution of Mn or Co in a YSZ electrolyte, a portion of which is in contact with an air electrode composed of a perovskite type oxide containing Mn or Co. In all of these disclosures, YSZ is employed for the electrolyte material.
Any of the above-noted disclosures has some limited advantage. There are still substantial differences, however in crystal structure and composition between fluorite type YSZ, which is the electrolyte material, and the perovskite type oxide, which is the air electrode material. For this reason, when the intermediate layer or the mixed layer is interposed between the two layers, a composition discontinuity, wherein a stepwise change in composition occurs, thereby reducing the effectiveness of the techniques to marginal improvements. The gradient composition layer cannot fully control the formation of the composition discontinuity. Each of the above disclosures employs the flame spraying process, supplying two types of powder, one for an electrolyte layer and the other for an air electrode layer, in a manner such that the ratio of mixture is gradually changed to form a composition gradient. If viewed from a micron-order scale, two types of particles having different compositions and different crystal structures are merely mixed. Accordingly, the control of the composition is difficult, and the products are thus subject to a reliability problem.